1. Field of the Invention
In recent years, vigorous developments have taken place in the field of optical disk apparatuses for use as recording devices having large capacities. The data format used for recording and reproducing data in an optical disk apparatus is generally classified into two types, a continuous composite servo format type and a Discrete Block servo Format type (hereinafter to be abbreviated as DBF), derived from the detection of a tracking signal of a light spot and a groove configuration on a disk medium. The present invention relates to an equalizer of an optical disk apparatus employing the DBF and to an optical disk recording/reproducing apparatus incorporated with such an equalizer.
2. Description of the Prior Art
Hitherto, DBF has been characterized by its relative ease of tracking signal detection and by the clock detection stability of the recorded and reproduced data, where all timings are detected using clock pits written on the disk.
Referring to the attached drawings, an explanation is given below of the signal reproducing portion of the conventional optical disk recording/reproducing apparatus.
FIG. 1 shows the signal reproducing portion of the conventional magnetooptical disk; FIG. 2 and FIG. 3 are waveform diagrams for illustrating the operation thereof; and FIG. 4 is a circuit diagram of the equalization circuit of the signal reproducing portion of the conventional magnetooptical disk.
In FIG. 1, reference numeral 1 depicts an optical head. The optical head is composed of a laser, a lens, an actuator, a polarized beam splitter, a pin photodiode, a prepit signal reproducing circuit, a magnetooptical signal reproducing circuit, etc. Reference numeral 70 denotes an optical disk having a track 71. Reference numerals 2 and 3 denote peak hold circuits, 4 denotes a subtractor, 5 denotes a differentiator, 6 denotes a PLL, 7 denotes a counter, 8 denotes a switch, 9 denotes an equalization circuit, 10 denotes an address decoder, and 11 denotes a system controller. Reference numeral 72 denotes a clock reproducer composed of the differentiator 5, the phase locked loop circuit 6, and the counter 7. Reference numeral 33 denotes a servo signal extractor composed of the peak hold circuits 2, 3 and the subtractor 4.
In FIG. 4, reference numeral 12 denotes an analog to digital converter, reference numerals 13-15 denote D flip-flops (hereinafter to be abbreviated as D-FFS), 16-18 denote multipliers, and reference numerals 19 and 20 denote adders.
The optical head 1 outputs a focus error signal in accordance with an optical spot formed on the magnetooptical disk 70 and outputs addresses, wobble marks and clock-pits unevenly recorded, as shown in FIG. 2, as pre-recording signals in accordance with a change in reflected light intensity.
The data recording and reproducing format of a 3.5 inch magnetooptical disk is explained with reference to FIG. 2 showing an enlargement of the track 71 of the disk 70 shown in FIG. 1. As shown in FIGS. 2(a)-(c), the data recording and reproducing format in the 3.5 inch magnetooptical disk is formed by dividing one circular track into 22 sectors, each sector comprising 76 blocks. Among the 76 blocks, the 0th block thru second block are the header blocks and include addresses of the sector mark, sector number, and track number. The third block thru 75th block are the data blocks, each data block comprising two servo-bytes and eight data bytes. The servo-bytes are composed of wobble marks for tracking error detection and clock-pits for clock reproduction, as shown in FIG. 2(c). Referring to FIG. 1, the differentiator 5 differentiates the input clock-pit signal and outputs it. The phase locked loop circuit 6, which has a free running frequency which is 110 times the repeating frequency of the clock-pit signal, accepts the differentiator 5 output only in the vicinity of the reproduction timing of the clock-pit signal in accordance with indication signal of the counter 7, and performs phase comparison of the reproduced clock output with the signal obtained by dividing the reproduced clock output into 110 sections using the counter 7 and synchronizes the same. The counter 7 is a 110 divisional counter, as mentioned above, which divides the aforementioned reproduced clock into 110 sections and at the same time outputs a timing indication signal of the clock pit and a timing indication signal of the wobble marks. By the above-mentioned operations, the clock reproducer 72 reproduces the clock.
The peak hold circuits 2 and 3 hold as outputs the maximum values V1 and V2 as in FIG. 2(d) of the wobble mark signals in accordance with the timings indicated by the counter 7, and the subtractor 4 outputs the difference between the peak hold circuits 2 and 3 as a tracking error signal. In this manner, the servo signal extractor 88 outputs the tracking error signal.
The address signal and the magnetooptical signal are recorded and reproduced in 4-11 modulation. In the 4-11 modulation, the data in the length of 11 bits is allocated to one symbol of a length of 1 byte before modulation, and its rule modulation is such that the number of the code "1" after the modulation becomes four in the one symbol of 11 bits. Accordingly, to 8 bytes of the data section in one block, the data of 88 bits are recorded and reproduced. This state corresponds to 110 bits in length of one block.
The write-in clock to be used for recording the magnetooptical signal is a reproduced clock reproduced from the aforementioned clock-pit, and similarly, for equalization and identification of data, the aforementioned reproduced clock is used. The address signal and the data signal outputted from the optical head 1 are applied to the switch 8. The switch 8 changes over between the address signal and the data signal in response to a switching signal generated by the system controller 11.
A circuit diagram of the equalization circuit 9 is shown in FIG. 4. The output of the switch 8 is inputted to the analog to digital (AD) converter 12. The AD converter 12 converts the analogue address signal and optical magnetic signal into digital signals. The sampling signal used is the reproduced clock of the phase locked loop circuit 7. The clocks applied to D-FFS 13-15 are also the aforementioned reproduced clock. The D-FFS 13-15 generate the sequentially one reproduced clock cycle delayed outputs of AD converter 12 to the multipliers 16-18. The multipliers 16-18 multiply the thus delayed signals by coefficients W(-1)-W(1) and output the multiplied results to the adders 19 and 20. D-FFs 13-15, multipliers 16-18, adders 19 and 20 constitute a 3-tap transversal filter having a transfer function H1 expressed as: EQU H1=W(-1).multidot.X(n+1)+W(0).multidot.X(n)+W(1).multidot.X(n-1)
The transfer characteristics vary according to the coefficients of the multiplier 16-18. The equalization circuit 9 has the effect of suppressing wave interference of adjacent bits of the reproduced signal S6-S8 as shown in FIG. 3(a). The address decoder 10 detects the address signal from the equalization circuit 9 and demodulates the current address. The system controller 11 receives the output of the address demodulator 10 and generates the changeover signal to the switch 8 for changing over between the address signal and the data signal.
One characteristic of the optical disk resides in its large memory capacity. Due to the fact that the linear density is equivalent to that of the magnetic disk apparatus and is more than 20 Kbpi, and due to the fact that the inter-track pitch of the recording track is as small as about 1.6 microns, a large recording density can be obtained. However, a demand for even higher density recording has arisen. To satisfy this demand, the linear density can be increased and the resultant wave interference reduced using the equalization circuit of the conventional data reproducing device. Further, it may also be considered to narrow the track pitch to thereby increase the recording density. However, when the track pitch is 1 micron or less, the crosstalk from adjacent tracks increases. The equalization circuit of the signal reproduction portion of the conventional magnetooptical disk apparatus, however, does not at all address the interference received from adjacent tracks, and thus limits the attainment of a high recording density.